1. Field
The following description relates to an apparatus for receiving optical signals, and more particularly, to an apparatus for receiving optical signals and a method of controlling a phase offset for differential quadrature phase shift keying.
2. Description of the Related Art
Differential quadrature phase shift keying (DPQSK) is a digital modulation scheme that conveys data by changing the phase of an optical signal and has an advantage of ensuring a secure long distance transmission over on-off keying in which data is conveyed by changing the intensity of an optical signal.
In order to receive a phase-modulated optical signal, a photodetector of a receiving end needs to be provided at a front end thereof with an optical delay interferometer that converts the phase-modulated signal into an intensity-modulated optical signal.
Transfer characteristics of an optical delay interferometer are dependent upon input wavelength. Accordingly, a phase difference between a delay signal and a non-delay signal needs to be appropriately set such that the phase-modulated signal is converted into the intensity-modulated signal.